The present invention relates to an instrument for testing a lubricating ability of a lubricating oil.
FIG. 8 shows a conventional test instrument (as disclosed in a U.S. Pat. No. 3,739,265).
A test instrument 1 of FIG. 8 includes a ring-shaped insulating plate 2, a central electrode 3, and a ring-shaped outer electrode 4. These members are so shaped as to form a liquid resorvoir 5. The bottom of the reservoir 5 is formed by the central electrode 3 and the insulating member 2, and the side wall of the reservoir 5 is formed by the outer electrode 4. The central and outer electrodes 3 and 4 are separated by the insulating member 2, and forms a capacitor, whose capacitance C is measured by a bridge circuit connected with both electrodes by leads 6 and 7. The material contained in the reservoir 5 serves as the dielectric of the capacitor, and the capacitance C is varied by the property of the material. When the lubricating oil is oxidized, or contaminated with sludge, dust, soot, acide etc., then the capacitance C becomes high. When the lubricating oil contains water, coolant or metal particles, then the capacitance becomes extremely high. When gasoline is mixed, the capacitance C becomes low. Therefore, this instrument makes it possible to know the degree of degradation of a lubricating oil by comparing a current capacitance value C.sub. x with an initial capacitance value C.sub.o which was obtained when the oil was new.
However, this conventional instrument is not capable of measuring an absolute value of a lubricating ability. This instrument is arranged to determine the degree of degradation of the oil by measuring a variation from the initial value.